Remove AbstractIndexSet typing

[gdalle]

Remove constraint S <: AbstractSet{<:Integer} in case we want to use vectors or other stuff.

Try to document the expectation of the tracer with respect to the "set" type.

See https://discourse.julialang.org/t/best-data-structure-for-fast-unions-of-large-sets-of-integers/113785/

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.57143% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 71.48%. Comparing base (8bc5c4e) to head (da1d342).

Files	Patch %	Lines
src/tracers.jl	64.70%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   71.48%   71.48%           
=======================================
  Files           8        8           
  Lines         270      270           
=======================================
  Hits          193      193           
  Misses         77       77           

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[gdalle]

I think this is ready, but I would rename all the occurrences of the set type to S. At the moment there is a mix of S and T. Thoughts?

[adrhill]

Yes, this is a good idea. T for tracer types, S for set types.

[gdalle]

I have put S in the few places I had spotted T, there might be more out there but I suggest we merge